1. Field of the Invention
This invention relates generally to nuclear reactors, and more particularly, to a control rod assembly of a liquid metal-cooled fast breeder nuclear reactor.
2. Description of the Prior Art
A nuclear reactor is designed and operated for the purpose of initiating and maintaining a nuclear fission chain reaction in a fissile material for the generation of heat for power purposes.
In the type of nuclear reactor described herein, fissile materials, such as plutonium-239 and uranium-238, are contained within fuel assemblies. A plurality of fuel assemblies comprise a nuclear core which is structurally supported within a hermetically sealed pressure vessel. A reactor coolant, such as liquid sodium, is circulated into the reactor pressure vessel and through the nuclear core where the heat generated by nuclear fission is transferred from the fuel assemblies to the reactor coolant. This heat is eventually transformed into steam which is then converted into electrical energy by means of conventional steam generator, steam turbine and electrical generator apparatus.
Control of the nuclear fission chain reaction, or more simply, control of the reactor is usually accomplished by control rod assemblies made up of neutron absorbing materials, such as boron carbide or tantalum. The control rod assemblies may be in the form of annular rings, clusters of rods or singular rods interdispersed throughout the nuclear core. The effectiveness of the control rod assemblies is determined by the amount of neutron absorbing or poison material introduced into the nuclear core. Withdrawing a control rod increases the nuclear reaction, while inserting a control rod decreases the nuclear reaction. At the beginning of a fuel cycle, the control rods are fully inserted. As the nuclear fuel burns up, the control rods are withdrawn to compensate for the decrease in nuclear activity.
The cooling requirements of the control rods depend upon their position with reference to the nuclear core. The heat generation rate within the control rods is highest when the rods are fully inserted. In the past, this position has been used as a reference to size orifices in order to assure adequate coolant flow through the control rod assemblies. However, at less than maximum heat generation, corresponding to less than full insertion, fixed area orificing causes over-cooling of the control rods, giving rise to uneven temperature distributions and unfavorable thermal stresses. The cooler exit temperature of the control rod coolant also lowers the overall temperature rise of the nuclear core reactor coolant which lowers the power plant efficiency. Thus, fixed area orificing of the control rod coolant flow of the prior art results in severe reactor penalties.